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Fees Paid by the Fund and Related Party Transactions (Additional Information) (Details) (USD $) | 3 Months Ended
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Mar. 31, 2015 | Dec. 31, 2014 | Mar. 31, 2014
Fees Paid and Related Party Transactions [Line Items] |  |  |
USCF Management Fee | USL is contractually obligated to pay USCF a fee, which is paid monthly, equal to 0.60% per annum of average daily total net assets. |  |
Estimated directors' fees and expenses | $ 2,905 us-gaap_NoninterestExpenseDirectorsFees |  | $ 4,177 us-gaap_NoninterestExpenseDirectorsFees
Licensing fee incurred | 2,320 us-gaap_LicenseCosts | 2,161 us-gaap_LicenseCosts | 2,161 us-gaap_LicenseCosts
Estimated investor tax reporting cost | 175,000 us-gaap_OtherGeneralExpense |  |
USL and the Related Public Funds, other than USCI, USAG, CPER and USMI |  |  |
Fees Paid and Related Party Transactions [Line Items] |  |  |
Estimated directors' fees and expenses | 560,625 us-gaap_NoninterestExpenseDirectorsFees / invest_InvestmentIdentifierSchemeAxis = usl_FundTrustsMember |  |
USL |  |  |
Fees Paid and Related Party Transactions [Line Items] |  |  |
Estimated directors' fees and expenses | $ 12,759 us-gaap_NoninterestExpenseDirectorsFees / invest_InvestmentIdentifierSchemeAxis = usl_UnitedStatesTwelveMonthOilFundLimitedPartnershipMember |  |
Licensing Agreements | On and after October 20, 2011 |  |  |
Fees Paid and Related Party Transactions [Line Items] |  |  |
Fee percentage | 0.015%usl_BaseFeePercentage / invest_InvestmentIdentifierSchemeAxis = usl_GroupThreeMember / us-gaap_TypeOfArrangementAxis = us-gaap_LicensingAgreementsMember |  |
Assets basis for determining fee percentage | on all net assets |  |

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- Definition Costs incurred and are directly related to generating license revenue. Licensing arrangements include, but are not limited to, rights to use a patent, copyright, technology, manufacturing process, software or trademark. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.2(e)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
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Balance Type: | debit
Period Type: | duration

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- Definition Description of the nature of payments to managing member or general partner for management of the day-to-day business functions of the limited liability company (LLC) or limited partnership (LP), including the fee rate, basis of calculation, relevant accounting period, whether the fee is paid to an entity other than the managing member or general partner, or whether the fee is waived. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Noninterest expense related to directors' fees which are fees paid by an Entity to its directors. Directors' fees may be paid in addition to salary and other benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.14) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | debit
Period Type: | duration

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- Definition Amount of general expenses not normally included in Other Operating Costs and Expenses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.6) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 5 + Details
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Balance Type: | debit
Period Type: | duration

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- Definition Base Amount For Fee Percentage + References No definition available. + Details
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- Definition Base Fee Percentage + References No definition available. + Details
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- Details
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- Details
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Namespace Prefix: |
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Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |